Master cylinder apparatus

ABSTRACT

A master cylinder apparatus includes: an input piston that can be moved forward by operating a brake operating member; a pressure piston that is provided in front of the input piston to be capable of moving relative to the input piston; and a stroke velocity ratio modification device that is capable of modifying a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages while the input piston moves from a rear end portion position to a front end portion position, and that sets the stroke velocity ratio at 1 when an abnormality occurs—in the master cylinder apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a master cylinder apparatus that has a mastercylinder and is included in a hydraulic brake system.

2. Description of Related Art

Japanese Patent Application Publication No. 2008-24098 (JP 2008-24098 A)discloses a master cylinder apparatus. The master cylinder includes aninput piston and a pressure piston, and the pressure piston can becaused to advance relative to the pressure piston by fluid pressure froma rearward back surface chamber.

SUMMARY OF THE INVENTION

The present invention provides an improved master cylinder apparatushaving a master cylinder that includes an input piston and a pressurepiston. According to the invention, a brake operating member operated bya driver, for example, is improved in operability.

A master cylinder apparatus according to a first aspect of the inventionincludes: an input piston that is configured to move forward byoperating a brake operating member; a pressure piston that is providedin front of the input piston configured to move relative to the inputpiston; and a stroke velocity ratio modification device that isconfigured to modify a stroke velocity ratio, which is a ratio between astroke velocity of the pressure piston and a stroke velocity of theinput piston, in at least two stages while the input piston moves from arear end portion position to a front end portion position, and that setsthe stroke velocity ratio at 1 when an abnormality occurs in the mastercylinder apparatus.

In the aspect described above, the stroke velocity ratio modificationdevice may be configured to modify the stroke velocity ratio in the atleast two stages when the stroke velocity ratio equals or exceeds 1.

In the aspect described above, the stroke velocity ratio modificationdevice may include a normal use region velocity ratio reduction unitthat sets the stroke velocity ratio small when a stroke of the inputpiston is large, compared to the stroke velocity ratio when the strokeof the input piston is small. In an initial stage of a brake operation,the stroke velocity ratio is increased, and therefore an initialresponse delay in the brake can be suppressed favorably.

In the aspect described above, the input piston may be disposed oppositethe pressure piston via an inter-piston chamber, the pressure piston mayinclude a large diameter portion, and a front small diameter portionthat is provided in front of the large diameter portion, and has asmaller diameter than the large diameter portion, and a surface area ofthe large diameter portion of the pressure piston on which pressure isreceived from a front side may be equal to or smaller than a surfacearea on which pressure is received from the inter-piston chamber side.When the pressure piston is caused to advance by fluid pressure in aback surface chamber in a condition where the opposing chamber isconnected to the inter-piston chamber but cut off from a reservoir,working fluid is supplied from the opposing chamber into theinter-piston chamber. When, in this case, an effective pressurereceiving surface area a1 of a part of the pressure piston that receivesfluid pressure from the opposing chamber is identical to an effectivepressure receiving surface area a2 of a part of the pressure piston thatreceives fluid pressure from the inter-piston chamber (a1=a2),theoretically the input piston is not caused to advance. As a result,the stroke velocity ratio takes an extremely large value, or in otherwords is theoretically infinite. When the effective pressure receivingsurface area a2 of the part of the pressure piston that receives fluidpressure from the inter-piston chamber is larger (a2>a1), on the otherhand, advancement of the input piston is permitted, and therefore thestroke velocity ratio takes a finite large value. In both cases, thestroke velocity ratio increases in an initial stage of a brakeoperation, and therefore an initial response delay in a brake can besuppressed favorably. Note that the effective pressure receiving surfacearea is the surface area of a part that actually receives fluidpressure, and takes a value (q/s) obtained by dividing volumetric changeq in a space inside the inter-piston chamber (the opposing chamber)capable of housing working fluid when the input piston (the pressurepiston) moves by a set stroke s by the set stroke.

In the aspect described above, the input piston may be disposed oppositethe pressure piston via the inter-piston chamber, and a surface area onwhich the input piston receives pressure from the inter-piston chambermay equal or exceed the surface area on which the pressure pistonreceives pressure from the inter-piston chamber. When an effectivepressure receiving surface area a3 of a part of the input piston thatreceives fluid pressure from the inter-piston chamber is larger than theeffective pressure receiving surface area a2 of the part of the pressurepiston that receives fluid pressure from the inter-piston chamber(a3>a2) and the inter-piston chamber is cut off from both the reservoirand the opposing chamber, a stroke velocity ratio (vout/vin) takes aninverse (a3/a2) of the effective pressure receiving surface area ratio,and therefore the stroke velocity ratio is 1 or more. Note that theeffective pressure receiving surface area a3 of the part of the inputpiston that receives the fluid pressure of the inter-piston chamber maybe set to be smaller than the effective pressure receiving surface areaa2 of the part of the pressure piston that receives the fluid pressureof the inter-piston chamber (a3<a2). In this case, the stroke velocityratio takes a smaller value than 1.

In the aspect described above, the input piston may be disposed oppositethe pressure piston via the inter-piston chamber, the pressure pistonmay include the large diameter portion, the front small diameter portionthat is provided in front of the large diameter portion and has asmaller diameter than the large diameter portion, and the stepconstituted by the large diameter portion and the front small diameterportion, and the surface constituting the step between the largediameter portion and the front small diameter portion of the pressurepiston may form an opposing chamber. Moreover, the master cylinderapparatus may further include a communication condition control deviceprovided between the inter-piston chamber, the opposing chamber, and areservoir and may be configured to control communication conditionsbetween the inter-piston chamber, the opposing chamber, and thereservoir so as to switch between an inter-chamber connection conditionin which the opposing chamber and the inter-piston chamber communicatewith each other but are cut off from the reservoir, an inter-chambercutoff condition in which the opposing chamber is cut off from theinter-piston chamber, the inter-piston chamber is cut off from thereservoir, and the opposing chamber communicates with the reservoir, anda reservoir connection condition in which both the opposing chamber andthe inter-piston chamber communicate with the reservoir. Thecommunication condition control device may include one or more solenoidcontrol valves or no solenoid control valves. A stroke velocity ratio γa(vout/vin), which is a ratio between a stroke velocity vout of thepressure piston and a stroke velocity yin of the input piston in theinter-chamber connection condition of (i), is determined by theeffective pressure receiving surface area a1 of the part of the pressurepiston that receives fluid pressure from the opposing chamber, theeffective pressure receiving surface area a2 of the part of the pressurepiston that receives fluid pressure from the inter-piston chamber, andthe effective pressure receiving surface area a3 of the part of theinput piston that receives fluid pressure from the inter-piston chamber.γa=a3/(a2−a1)

A stroke velocity ratio γb in the inter-chamber cutoff condition of (ii)is determined by the effective pressure receiving surface area a3 of thepart of the input piston that receives fluid pressure from theinter-piston chamber and the effective pressure receiving surface areaa2 of the pressure piston.γb=a3/a2

In the reservoir connection condition of (iii), the input piston and thepressure piston are caused to advance integrally, and therefore a strokevelocity ratio γc is 1 (γc=1). The reservoir connection condition can beset when the master cylinder apparatus is inoperative or an abnormalityoccurs in an electrical system.

In the aspect described above, the communication condition controldevice may include: an inter-chamber connection cutoff valve constitutedby a normally open solenoid valve provided between the opposing chamberand the inter-piston chamber; a reservoir connection valve constitutedby a normally open solenoid valve provided between the opposing chamberand the reservoir; and a solenoid valve control unit that controls thecommunication conditions between the opposing chamber, the inter-pistonchamber, and the reservoir by controlling the reservoir connection valveand the inter-chamber connection cutoff valve. The communicationconditions between the opposing chamber, the inter-piston chamber, andthe reservoir are controlled by controlling opening and closing of thereservoir connection valve and the inter-chamber connection cutoffvalve. However, when an abnormality preventing solenoid valve controloccurs, no current is supplied to the solenoids. As a result, thereservoir connection valve and the inter-chamber connection cutoff valveare opened such that the opposing chamber and the inter-piston chambercommunicate with each other and with the reservoir.

In the aspect described above, the communication condition controldevice may include: a connection cutoff mechanism that is providedbetween the opposing chamber and the inter-piston chamber and reservoirand includes a movable member operated by a pilot pressure, a magnitudeof which is determined through electric control, and configured to movebetween a communication position in which the opposing chamber and theinter-piston chamber communicate with each other but are cut off fromthe reservoir, and a cutoff position in which the opposing chambercommunicates with the reservoir but the inter-piston chamber is cut offfrom both the opposing chamber and the reservoir; and a reservoirconnection valve constituted by a normally open solenoid valve providedbetween the opposing chamber and the reservoir. In a case where themovable member of the connection cutoff mechanism is switched from thecommunication position to the cutoff position when the pilot pressureincreases beyond a set pressure, the movable member is held in thecommunication position after an abnormality occurs in the electricalsystem such that the pilot pressure cannot be increased beyond the setpressure. Further, when an abnormality occurs in the electrical system,the reservoir connection valve opens. As a result, the opposing chamberand the inter-piston chamber communicate with each other and communicatewith the reservoir via the reservoir connection valve.

A master cylinder apparatus according to a second aspect of theinvention includes: an input piston that is configured to move forwardby operating a brake operating member; and a pressure piston that isprovided coaxially with the input piston and configured to move relativeto the input piston, disposed opposite the input piston via aninter-piston chamber, and has a stepped shape including a large diameterportion and a front small diameter portion that has a smaller diameterthan the large diameter portion and is provided in front of the largediameter portion, wherein a communication condition control device isprovided between an opposing chamber, which is provided in front of astep surface between the large diameter portion and the front smalldiameter portion, the inter-piston chamber and a reservoir, thecommunication condition control device being configured to switchcommunication conditions among the opposing chamber, the inter-pistonchamber, and the reservoir between at least a condition in which theopposing chamber and the inter-piston chamber communicate with eachother but are cut off from the reservoir, a condition in which theopposing chamber is cut off from the inter-piston chamber, the opposingchamber communicates with the reservoir, and the inter-piston chamber iscut off from the reservoir, and a condition in which both the opposingchamber and the inter-piston chamber communicate with the reservoir.

In the aspect described above, the master cylinder apparatus may furtherinclude a back surface pressure control device having a power pressuresource that is operated by a supply of power and is configured to outputa fluid at a predetermined pressure, and a regulator that controls apressure exerted on a back surface chamber, which is provided rearwardof a pressure receiving surface of the pressure piston, to a magnitudecorresponding to an operating condition of the brake operating memberusing the pressure output by the power pressure source. The pressurepiston is caused to advance by fluid pressure in the back surfacechamber, and therefore, by controlling the fluid pressure in the backsurface chamber to a magnitude corresponding to the operating conditionof the brake operating member, fluid pressure in a pressure chamber canalso be controlled to a magnitude corresponding to the operatingcondition of the brake operating member. The operating condition of thebrake operating member may be represented by at least one of anoperating force applied to the brake operating member and an operatingstroke thereof. Further, when an abnormality occurs in the electricalsystem, high-pressure fluid pressure can no longer be output from thepower fluid pressure source to the regulator, and it is often, as aresult, impossible to control the fluid pressure in the back surfacechamber.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a view showing a hydraulic brake system including a mastercylinder apparatus according to a first embodiment of the invention;

FIG. 2 is a sectional view showing a regulator of a servo pressuresupply device included in the master cylinder apparatus;

FIG. 3 is a view showing a relationship between a servo pressure of theregulator and a brake operating force;

FIG. 4 is a view showing a relationship between respective strokes of aninput piston and a pressure piston of a master cylinder included in themaster cylinder apparatus;

FIG. 5A is a view showing a condition of a communication conditioncontrol device included in the master cylinder apparatus;

FIG. 5B is a flowchart illustrating a solenoid valve control programstored in a storage unit of a brake electronic control unit (ECU)included in the hydraulic brake system;

FIG. 6 is a view showing a hydraulic brake system including a mastercylinder apparatus according to a second embodiment of the invention;

FIG. 7A is a conceptual diagram (a partial sectional view) showing acommunication cutoff control device of the hydraulic brake system;

FIG. 7B is a view showing a condition of a communication conditioncontrol device according to the second embodiment of the invention;

FIG. 8 is a view showing a hydraulic brake system including a mastercylinder apparatus according to a third embodiment of the invention;

FIG. 9A is a conceptual diagram (a partial sectional view) showing acommunication cutoff control device of the hydraulic brake system;

FIG. 9B is a view showing a condition of the communication conditioncontrol device;

FIG. 10 is a view showing a hydraulic brake system including a mastercylinder apparatus according to a fourth embodiment of the invention;

FIG. 11A is a conceptual diagram (a partial sectional view) showing acommunication cutoff control device of the hydraulic brake system;

FIG. 11B is a view showing a condition of the communication conditioncontrol device;

FIG. 12 is a view showing a hydraulic brake system including a mastercylinder apparatus according to a fifth embodiment of the invention;

FIG. 13A is a view (a partial sectional view) showing a servo pressuresupply device included in the master cylinder apparatus;

FIG. 13B is a view showing a relationship between a target value of aservo pressure of the servo pressure supply device and the brakeoperating force;

FIG. 14 is a view showing the hydraulic brake system including themaster cylinder apparatus according to the fifth embodiment of theinvention;

FIG. 15 is a view showing a relationship between respective strokes ofan input piston and a pressure piston of the master cylinder apparatus;

FIG. 16 is a view showing a hydraulic brake system including a mastercylinder apparatus according to a sixth embodiment of the invention; and

FIG. 17 is a view showing a relationship between respective strokes ofan input piston and a pressure piston of the master cylinder apparatus.

DETAILED DESCRIPTION OF EMBODIMENTS

A hydraulic brake system including a master cylinder according to anembodiment of the invention will be described in detail below on thebasis of the drawings. The hydraulic brake system includes a mastercylinder apparatus according to an embodiment of the invention.

The hydraulic brake system is provided in a vehicle. FIG. 1 shows anexample of the hydraulic brake system according to the invention. Thehydraulic brake system includes (i) brake cylinders 12FL, 12FR, 12RL,12RR of hydraulic brakes that are provided respectively on front, rear,left, and right wheels 10FL, 10FR, 10RL, 10RR and operated by fluidpressure to suppress rotation of the respective wheels, (ii) a mastercylinder apparatus 13, and so on. The master cylinder apparatus 13includes (a) a master cylinder 14 that supplies fluid pressure to thebrake cylinders 12FL, 12FR, 12RL, 12RR, (b) a communication conditioncontrol device 15 that controls communication conditions between areservoir and an opposing chamber and an inter-piston chamber of themaster cylinder 14, to be described below, (c) a servo pressure supplydevice 18 serving as a back surface fluid pressure control device thatsupplies regulatory fluid pressure (also referred to as servo pressurehereafter) to a back surface chamber 16 of the master cylinder 14, andso on. Note that the communication condition control device 15 may beprovided separately to the master cylinder 14 or as part of theconstituent elements of the master cylinder 14.

The master cylinder 14 includes (1) a housing 20, and (2) an inputpiston 22 and two pressure pistons 24, 25 fitted to the housing 20 to befluid-tight and capable of sliding. The input piston 22 and the twopressure pistons 24, 25 are disposed on an identical axis (Lm) to becapable of moving relative to each other. A brake pedal 26 serving as abrake operating member is linked to the input piston 22 via an operationrod 27 to be capable of advancing in response to a depression operationof the brake pedal 26. Further, a return spring 27 r is provided betweena member capable of moving integrally with the input piston 22 and thehousing 20. Pressure chambers 28, 29 are formed respectively in front ofthe pressure pistons 24, 25. The brake cylinders 12FL, 12FR of the leftand right front wheels 10FL, 10FR are connected to the pressure chamber28, and the brake cylinders 12RL, 12RR of the left and right rear wheels10RL, 10RR are connected to the pressure chamber 29. Furthermore, returnsprings 29 r, 28 r are provided respectively between the pressurepistons 24, 25 and between the pressure piston 24 and the housing 20. Aninter-piston chamber 30 is provided between the pressure piston 25 andthe input piston 22 to the rear thereof. Hence, in the hydraulic brakesystem according to this embodiment, the master cylinder 14 is a tandemtype master cylinder having front and rear systems.

In the pressure piston 25, a front portion is constituted by a frontsmall diameter portion 32, an intermediate portion is constituted by anintermediate large diameter portion 33, and a rear portion isconstituted by a rear small diameter portion 34 having a smallerdiameter than the front small diameter portion 32. The pressure piston25 is formed in a stepped shape by the front small diameter portion 32and the intermediate large diameter portion 33. The pressure chamber 29is provided in front of the front small diameter portion 32. An opposingchamber 38 is formed in front of a step surface 36 between the frontsmall diameter portion 32 and the intermediate large diameter portion33. The back surface chamber 16 is provided rearward of a step surface42 serving as a pressure receiving surface between the intermediatelarge diameter portion 33 and the rear small diameter portion 34.Further, the front small diameter portion 32, the intermediate largediameter portion 33, and the rear small diameter portion 34 arerespectively fitted in a fluid-tight fashion to the housing 20. As aresult, the opposing chamber 38, the back surface chamber 16, theinter-piston chamber 30, and the pressure chamber 29 are cut off fromeach other so as to be fluid-tight. In other words, fluid pressure canbe generated individually and independently in each of the opposingchamber 38, the back surface chamber 16, the inter-piston chamber 30,and the pressure chamber 29.

In this embodiment, an effective pressure receiving surface area a1 (=a1x−a1 y) of the step surface 36 of the pressure piston 25 that opposesthe opposing chamber 38 is smaller than an effective pressure receivingsurface area a2 of a part of the rear small diameter portion 34positioned in the inter-piston chamber 30 (a1<a2), and the effectivepressure receiving surface area a2 is smaller than an effective pressurereceiving surface area a3 of a part of the input piston 22 positioned inthe inter-piston chamber 30 (a3>a2). The effective pressure receivingsurface area is a surface that substantially receives fluid pressure.More specifically, when a stroke of a piston (here, the pressure piston25 and the input piston 22) is set as s and volumetric change in a spaceof a fluid pressure chamber (here, the opposing chamber 38 and theinter-piston chamber 30) housing working fluid is set as q, theeffective pressure receiving surface area takes a value (q/s) obtainedby dividing the volumetric change q by the set stroke s.

The communication condition control device 15 controls communicationconditions between the inter-piston chamber 30, the opposing chamber 38,and a reservoir 50. The communication condition control device 15includes (i) a reservoir passage 54 connecting the opposing chamber 38to the reservoir 50, (ii) an inter-chamber connection passage 56connecting the opposing chamber 38 to the inter-piston chamber 30, (iii)a reservoir connection valve 58 provided in the reservoir passage 54,and (iv) an inter-chamber connection cutoff valve 60 provided in theinter-chamber connection passage 56. The reservoir connection valve 58and the inter-chamber connection cutoff valve 60 are respectivelyconstituted by normally open solenoid valves that are open when acurrent is not supplied to respective solenoids thereof.

The servo pressure supply device 18 includes a regulator 90, a highpressure source 92, a linear valve device 94, and so on. As shown inFIG. 2, the regulator 90 is capable of controlling the fluid pressure(servo pressure) supplied to the back surface chamber 16 to a magnitudecorresponding to an operating force (also referred to as a brakeoperating force hereafter) applied to the brake pedal 26 using fluidpressure from the high pressure source 92. The regulator 90 includes ahousing 100, a spool 102 fitted to the housing 100 to be capable ofsliding, an advancement driving member 104 that applies force to thespool 102 in an advancement direction, and a retreat driving member 106that applies force to the spool 102 in a retreat direction. The spool102, the advancement driving member 104, and the retreat driving member106 are respectively disposed on an identical axis (Ls) to be capable ofmoving relative to each other. The housing 100 is provided with anoutput port 110 to which the back surface chamber 16 is connected, aninput port 112 to which the inter-piston chamber 30 is connected, amaster pressure port 114 to which the pressure chamber 29 is connected,a low pressure port 118 to which the reservoir 50 is connected via apressure reducing linear valve 116, a high pressure port 120 to whichthe high pressure source 92 is connected, a linear pressure port 124 towhich the high pressure source 92 is connected via a pressure increasinglinear valve 122, and a feedback pressure port 126 to which the backsurface chamber 16 is connected. These ports are provided in the housing100 at intervals from each other in a radial direction or a direction ofthe axis (Ls). An annular communication groove 130 extending in the axis(Ls) direction is formed in an outer peripheral portion of anintermediate portion of the spool 102. The communication groove 130 isformed in a position and a size whereby the output port 110 and thelinear pressure port 124 are open normally, the low pressure port 118opens when the spool 102 is in a retreat end position, and the highpressure port 120 opens when the spool 102 is in an advancement endposition. Fluid pressure in the output port 110 is controlled by movingthe spool 102 relative to the housing 100 so that either the lowpressure port 118 or the high pressure port 120 is connected selectivelyto the output port 110. A return spring 132 is provided between thespool 102 and the housing 100 to bias the spool 102 in a retreatdirection. Further, a rear end surface 133 of the spool 102 receivesfluid pressure from the input port 112.

The advancement driving member 104 is disposed to the rear of the spool102, and fluid pressure from the master pressure port 114 is received bya rear end surface 134 thereof. The advancement driving member 104 canbe caused to advance by advancement direction force generated by thefluid pressure of the master pressure port 114, and applies theadvancement direction force generated by the master pressure to thespool 102. Further, the advancement driving member 104 has a steppedshape including a small diameter portion and a large diameter portion,and the retreat end position is defined by contact between a stepportion formed between the small diameter portion and the large diameterportion and the housing 100. In this condition, a front end surface ofthe advancement driving member 104 functions as a stopper thatdetermines the retreat end position of the spool 102.

The retreat driving member 106 is disposed in front of the spool 102 viaa gap, and fluid pressure in the feedback pressure port 126 is receivedby a front end surface 136 thereof. An elastic member 140 made of rubberor the like is provided on a rear portion (a main body rear portion) ofthe retreat driving member 106, and a retainer 141 having a stopperfunction is provided in an intermediate portion so as to project in theradial direction. The advancement end position is defined by contactbetween the retainer 141 having a stopper function and the housing 100.Meanwhile, a return spring 142 is provided between the retainer 141having a stopper function and the housing 100. The return spring 142biases the retreat driving member 106 in the advancement direction. Aset load Fset of the return spring 142 is set at a comparatively largevalue. The retreat driving member 106 can be caused to retreat byretreat direction force of a magnitude obtained by subtracting anelastic force of the return spring 142 from the fluid pressure of thefeedback pressure port 126, and applies the retreat direction force tothe spool 102.

The spool 102, the advancement driving member 104, and the retreatdriving member 106 are respectively fitted to the housing 100 to befluid-tight. As a result, the master pressure port 114, the input port112, and the feedback pressure port 126 are cut off from each other in afluid-tight manner. Further, a surface area of the rear end surface 133of the spool 102 is set as Aio, a surface area of a part 144 obtained bysubtracting a surface area of a contact portion contacting theadvancement driving member 104 from the rear end surface 133 (a surfacearea of an annular part, or in other words a surface area of a part thatreceives the fluid pressure of the input port 112 in a condition wherethe spool 102 contacts the advancement driving member 104) is set as Ai,a surface area of the rear end surface 134 of the advancement drivingmember 104 is set as Am, and a surface area of the front end surface 136of the retreat driving member 106 is set as As. Furthermore, in acondition where the spool 102 is in the retreat end position (the spool102 is located at a rearward end portion of a movable range thereof) andthe retreat driving member 106 is in the advancement end position (arearward end portion of a range in which the retreat driving member 106can move toward the spool 102), a gap x1 between a rear end surface ofthe elastic member 140 provided on the retreat driving member 106 and afront end surface of the spool 102 equals or exceeds a distance x2between a rear end surface of the communication groove 130 and the lowpressure port 114 (x1≥x2), and a gap x3 between a main body rear endsurface 146 of the retreat driving member 106 and the front end surfaceof the spool 102 equals or exceeds a distance x4 between a front endsurface of the communication groove 130 of the spool 102 and the highpressure port 112 (x3≥x4), wherein the distance x1 is equal to orshorter than the distance x4 (x1≤x4). The distances x1 to x4 aredesigned so that the spool 102 can move to a pressure increasingposition in which the output port 110 communicates with the highpressure part 120 via the communication groove 130 before the front endsurface of the spool 102 contacts the main body rear end surface 146 ofthe retreat driving member 106, and so that in the pressure increasingposition, the spool 102 contacts (and, in certain cases, elasticallydeforms) the elastic member 140.

The high pressure source 92 includes a pump device 163 having a pump 160and a pump motor 162, an accumulator 164, and an accumulator pressuresensor 166 that detects fluid pressure in the accumulator 164. The pump160 is a plunger pump, for example. The pump motor 162 is controlled tokeep the accumulator pressure within a set range. As described above,the linear valve device 94 includes the pressure increasing linear valve122 provided between the high pressure source 92 and the linear pressureport 124, and the pressure reducing linear valve 116 provided betweenthe low pressure port 118 and the reservoir 50. Respective front-reardifferential pressures of the pressure increasing linear valve 122 andthe pressure reducing linear valve 116 can be controlled to magnitudescorresponding to amounts of current supplied to respective solenoidsthereof. Further, the pressure increasing linear valve 122 and thepressure reducing linear valve 116 are normally open valves that areopen when no current is supplied to the solenoids. The linear valvedevice 94 is used during an automatic brake operation such that when thebrake pedal 26 is operated, the pressure increasing linear valve 122 iskept closed and the pressure reducing linear valve 116 is kept open.Note that the pressure increasing linear valve 122 may be a normallyclosed valve.

Furthermore, a slip control valve device 182F including at least onesolenoid valve is provided between the pressure chamber 28 and the brakecylinders 12FL, 12FR of the left and right front wheels. Moreover, aslip control valve device 182R including at least one solenoid valve isprovided between the pressure chamber 29 and the brake cylinders 12RL,12RR of the left and right rear wheels.

The hydraulic brake system is provided with a brake ECU 200 (see FIG. 1)having a computer as a main body. The brake ECU 200 includes anexecution unit, an input/output unit, and a storage unit. Theaccumulator pressure sensor 166, a stroke sensor 210 that detects anoperating stroke of the brake pedal 26, a depression force sensor 212that detects a depression force as the operating force applied to thebrake pedal 26, an input fluid pressure sensor 214 that detects thefluid pressure in the inter-piston chamber 30, and so on are connectedto the input/output unit together with the reservoir connection valve58, the connection cutoff valve 60, the linear valve device 94, the pumpmotor 162, and so on. A large number of programs and tables, including asolenoid valve control program, are stored in the storage unit of thebrake ECU 200.

An operation of this hydraulic brake system will now be described.

[Non-Brake Operation Condition]

When a depression operation has not been performed on the brake pedal 26(in a non-brake operation condition), the master cylinder 14, thecommunication condition control device 15, and the regulator 90 are inorigin positions shown in the drawing. In the master cylinder 14, theinput piston 22 and the pressure pistons 24, 25 are in the retreat endposition, whereby the inter-piston chamber 30 and the pressure chambers28, 29 communicate with the reservoir 50. In the regulator 90, theoutput port 110 communicates with the low pressure port 118, and theback surface chamber 16 communicates with the reservoir 50.

[Initial Stage of Brake Operation]

When the brake pedal 26 is depressed, the reservoir connection valve 58and the inter-chamber connection cutoff valve 60 of the communicationcondition control device 15 are respective set in a closed condition andan open condition, as shown in FIG. 5. In the master cylinder 14, theinput piston 22 advances, thereby cutting off the inter-piston chamber30 from the reservoir 50, and as a result, fluid pressure is generated.The fluid pressure of the inter-piston chamber 30 is supplied to theregulator 90.

In the regulator 90, the fluid pressure of the inter-piston chamber 30is supplied from the input port 112 such that advancement directionforce acts on the spool 102. When the advancement direction forceexceeds a set load of the return spring 132, the spool 102 advancesrelative to the advancement driving member 104. The output port 110 iscut off from the low pressure port 118 and connected to the highpressure port 120. As a result, fluid pressure starts to be supplied tothe back surface chamber 16 (a point As in FIG. 3). Since the highpressure port 120 communicates with the output port 110, the fluidpressure in the back surface chamber 16 increases on a large gradient ina region RAs in FIG. 3. A position in which the output port 110 and thehigh pressure port 120 of the spool 102 communicate is available as thepressure increasing position. As described above, x1≥x2, x3≥x4, andx4≥x1 are established, and therefore, when the advancement directionforce acting on the spool 102 equals or exceeds a sum (F1+F2) of a forceF1 by which the return spring 132 can be elastically deformed by adisplacement amount x4 and a force F2 by which the elastic member 140can be elastically deformed by a displacement amount (x4−x1), the spool102 is moved to the pressure increasing position {when x4=x1, F2 iszero}. Further, in the pressure increasing position of the spool 102,the spool 102 contacts the elastic member 140. Note that in thisembodiment, the set load and a spring constant of the return spring 132and a set load and a spring constant of the elastic member 140 are setat small values, and therefore the spool 102 is moved to the pressureincreasing position when the advancement direction force acting on thespool 102, or in other words the fluid pressure in the inter-pistonchamber 30 (corresponding to the brake operating force) is small.

When the spool 102 is in the pressure increasing position, retreatdirection force Fb having a magnitude indicated by a following equationis applied to the retreat driving member 106 by a fluid pressure Ps ofthe back surface chamber 16.Fb=Ps×As−Pi×Aio  (1)

In the above equation, a fluid pressure Pi is the fluid pressure of theinter-piston chamber 30. The spool 102 contacts the retreat drivingmember 106, and therefore advancement direction force generated by thefluid pressure in the input port 112 acts on the elastic member 140 (theretreat driving member 106) via the spool 102. When the retreatdirection force Fb acting on the retreat driving member 106 exceeds theset load Fset of the return spring 142 (Fb>Fset), the retreat drivingmember 106 is moved in the retreat direction, and as a result, the spool102 retreats. The high pressure port 120 is disconnected from thecommunication groove 130, and the high pressure port 120 is cut off fromthe output port 110 (a point Bs in FIG. 3). A fluid pressure Psa of theback surface chamber 16 at this point has a magnitude indicated by afollowing equation.Psa=(Fsets+Pi×Aio)/As  (2)

Further, a brake operating force Fps at this point has a magnitudecorresponding to the fluid pressure Pi of the inter-piston chamber 30,and can be obtained in advance (hereafter, the operating force Fps willalso be referred to as an initial operation completion determinationoperating force Fpb).

In the master cylinder 14, when the advancement direction force actingon the pressure piston 25 exceeds a set load of the return spring 29 r,the pressure pistons 25, 24 start to advance (a point Af in FIG. 4).When the pressure pistons 25, 24 advance, the pressure chambers 29, 28are cut off from the reservoir 50, and as a result, fluid pressure isgenerated. Further, the opposing chamber 38 and the inter-piston chamber30 are in a communicative condition, and therefore, as the pressurepiston 25 advances, working fluid is supplied from the opposing chamber38 to the inter-piston chamber 30. In this embodiment, the effectivepressure receiving surface area a1 of the pressure piston 25 relative tothe opposing chamber 38 is smaller than the effective pressure receivingsurface area a2 thereof relative to the inter-piston chamber 30 (a1<a2),and therefore advancement of the input piston 22 is permitted even whenthe working fluid is supplied to the inter-piston chamber 30 from theopposing chamber 38. As shown in FIG. 4, in a region RAf, a ratio γa(=vin/vout) between a stroke velocity yin of the input piston 22 and astroke velocity vout of the pressure piston 25 takes a magnitudeexpressed by a following equation.γa=a3/(a2−a1)  (3)

This embodiment is designed such that a3>a2>a1 and a difference (a2−a1)is small. The ratio γa therefore takes a large value. Note that both aforce corresponding to the fluid pressure in the inter-piston chamber 30and a force corresponding to the fluid pressure in the back surfacechamber 16 are exerted on the pressure pistons 25, 24, and thereforefluid pressure corresponding to the advancement direction force actingon the pressure pistons 25, 24 is generated in the pressure chambers 29,28. This embodiment is designed such that when the fluid pressure in theback surface chamber 16 reaches the magnitude indicated by Equation (2),the fluid pressure of the pressure chambers 28, 29, or in other words afluid pressure of the brake cylinder 12, reaches a set pressure Pma thatequals or exceeds a fluid pressure at which a first fill is completed.

[Normal Use Region]

In the communication condition control device 15, as shown in FIG. 5,when the brake operating force Fp detected by the depression forcesensor 212 reaches the initial operation completion determinationoperating force Fps, the inter-chamber connection cutoff valve 60 isclosed and the reservoir connection valve 58 is opened. Note that acontrol timing of the communication condition control device 15 may bedetermined on the basis of the fluid pressure in the inter-pistonchamber 30, detected by the input fluid pressure sensor 214, and theoperating stroke of the brake pedal 26, detected by the stroke sensor210, instead of the brake operating force. The fluid pressure Pi of theinter-piston chamber 30, which corresponds to the servo pressure Psa,can be obtained from Equation (2). Further, the operating stroke of thebrake pedal 26, which corresponds to the initial operation completiondetermination operating force Fps, can be obtained in advance.

In the regulator 90, the fluid pressure in the pressure chambers 28, 29increases, and when a fluid pressure Pm supplied to the master pressureport 114 increases, the advancement driving member 104 advances so as tocontact the spool 102. In a condition where the spool 102, theadvancement driving member 104, and the retreat driving member 106 (theelastic member 140) contact each other, a force expressed by a followingequation acts on the spool 102.Ps×As−(Ks×Δ+Fsets)=Pi×Ai+Pm×Am  (4)

In the above equation, Pm is the fluid pressure of the pressure chamber29, Ks is a modulus of elasticity of the return spring 142, and Δ is adisplacement amount of the return spring 142. According to the aboveequation, when the retreat direction force on the left side and theadvancement direction force on the right side are counterbalanced, thespool 102 moves in the direction of the axis Ls such that the outputport 110 communicates selectively with the high pressure port 120 or thelow pressure port 118. As a result, an increase gradient of the servopressure Ps relative to the brake operating force Fp (corresponding tothe fluid pressure Pi of the inter-piston chamber 30 and the fluidpressure Pm of the pressure chamber 29) is smaller in a region RBs ofFIG. 3 than in the region RAs. In the master cylinder 14, theinter-piston chamber 30 is cut off from the opposing chamber 38 and thereservoir 50, whereas the opposing chamber 38 communicates with thereservoir 50. As indicated by a following equation, a stroke velocityratio γb (vout/vin) in this case is γb=a3/a2. The ratio γb is greaterthan 1. Note that since a force corresponding to the fluid pressure inthe inter-piston chamber 30 and a force corresponding to the fluidpressure in the back surface chamber 16 act on the pressure pistons 25,24, the magnitude of the fluid pressure in the pressure chambers 29, 28is determined by these forces. Meanwhile, the fluid pressure in the backsurface chamber 16 has a magnitude corresponding to the brake operatingforce, and therefore the fluid pressure in the pressure chambers 29, 28also has a magnitude corresponding to the brake operating force.

[When Abnormality Occurs in Electrical System]

In the communication condition control device 15, as shown in FIG. 5,when the current supply to the solenoids is stopped, the reservoirconnection valve 58 and the connection cutoff valve 60 are opened.Accordingly, both the opposing chamber 38 and the inter-piston chamber30 communicate with the reservoir 50. In the regulator 90, no fluidpressure is generated in the inter-piston chamber 30, and therefore, inthe initial stage of the brake operation, the spool 102 is in theposition shown in the drawing. When fluid pressure is subsequentlygenerated in the pressure chamber 28 such that the advancement directionforce increases, the advancement driving member 104 advances, therebycausing the spool 102 to advance. The output port 110 is cut off fromthe low pressure port 118 and connected to the high pressure port 120.The fluid pressure of the output port 110 is controlled while fluidpressure remains in the accumulator 164, and therefore the servopressure Ps can be supplied to the back surface chamber 16. Further,even when fluid pressure can no longer be supplied from the accumulator164, working fluid can be supplied from the reservoir 50 to the outputport 110 via the high pressure port 120 and the linear valve port 124(the pressure increasing linear valve 122 of which is open) by an actionof a check valve (a discharge valve, an intake valve) provided in theplunger pump 160. In the master cylinder 14, when the brake pedal 26 isdepressed (caused to perform an advancement operation), the input piston22 advances so as to contact the pressure piston 25. The input piston 22and the pressure piston 25 advance integrally, and therefore a strokevelocity ratio γc is 1. Further, by supplying the servo pressure Ps tothe back surface chamber 16, the fluid pressure in the pressure chambers28, 29 can be increased correspondingly.

[Execution of Solenoid Valve Control Program]

The reservoir connection valve 58 and the inter-chamber connectioncutoff valve 60 of the communication condition control device 15 arecontrolled by executing a solenoid valve control program illustrated ona flowchart shown in FIG. 5B. In step 1 (abbreviated hereafter to S1;likewise for all other steps), a determination is made as to whether ornot an operation to depress the brake pedal 26 has been performed. Inthis embodiment, a depression operation can be detected by determiningwhether or not a detection value of the stroke sensor 210 equals orexceeds an operation start threshold (a stroke) at which it may bedetermined that the brake pedal 26 has been depressed; whether or not adetection value of the depression force sensor 212 equals or exceeds anoperation start threshold (an operating force) at which it may bedetermined that the brake pedal 26 has been depressed, and so on.Further, a brake switch may be provided, and the depression operationmay be detected on the basis of an ON/OFF condition of the brake switch.When the depression operation of the brake pedal 26 is not detected, acurrent is not supplied to the solenoids of the reservoir connectionvalve 58 and the inter-chamber connection cutoff valve 60 in S2. Hence,the reservoir connection valve 58 and the inter-chamber connectioncutoff valve 60 are kept open. When the depression operation of thebrake pedal 26 is detected, a determination is made in S3 as to whetheror not the detection value of the depression force sensor 212 equals orexceeds the initial operation completion determination operating forceFps. When the detection value is smaller than the initial operationcompletion determination operating force Fps, the reservoir connectionvalve 58 is closed and the connection cutoff valve 60 is opened in S4.This condition is maintained as long as the brake operating forceremains smaller than the initial operation completion determinationoperating force Fps, and when the brake operating force reaches orexceeds the initial operation completion determination operating forceFps, the reservoir connection valve 58 is opened and the connectioncutoff valve 60 is closed in S5. Note that when an abnormality occurs inthe electrical system, a current is not supplied to the solenoids, andtherefore the reservoir connection valve 58 and the inter-chamberconnection cutoff valve 60 remain open. Hence, in this embodiment, thecommunication conditions between the inter-piston chamber 30, theopposing chamber 38, and the reservoir 50 are controlled by controllingthe two solenoid valves 58, 60.

[During Automatic Brake Operation]

When it is necessary to operate an automatic brake, for example duringtraction control, vehicle stability control, inter-vehicle control, andso on, the linear valve device 94 (the pressure increasing linear valve122 and the pressure reducing linear valve 116) of the servo pressuresupply device 18 is controlled. The fluid pressure controlled by thelinear valve device 94 is supplied to the back surface chamber 16 viathe output port 110, and as a result, the pressure pistons 25, 24advance relative to the input piston 22 such that fluid pressure isgenerated in the pressure chambers 29, 28.

According to this embodiment, therefore, the stroke velocity ratio inthe master cylinder 14 while the brake pedal 26 moves from the retreatend position to the advancement end position takes a value largerthan 1. As a result, an operating stroke by which a driver operates thebrake pedal 26 can be reduced. Further, the stroke velocity ratio can bemodified in at least two stages, i.e. the initial stage of the brakeoperation and the normal use region, and therefore the stroke velocityratio is greater in the initial stage of the brake operation than in thenormal use region. As a result, the operating stroke in the initialstage of the brake operation can be reduced favorably while favorablysuppressing an initial response delay. Furthermore, by adjusting theoperating stroke in the normal use region, the fluid pressure of thepressure chambers 28, 29 can be regulated easily, leading to animprovement in an operating feeling experienced by the driver. Moreover,when an abnormality occurs in the electrical system, the inter-pistonchamber 30 and the opposing chamber 38 can both be connected to thereservoir 50, and in so doing, the stroke velocity ratio can be setat 1. As a result, an increase in the operating stroke of the driver canbe suppressed even when an abnormality occurs in the electrical system.

As is evident from the above description, a stroke velocity ratiomodification device is constituted by the communication conditioncontrol device 15, the pressure piston 25, the input piston 22, parts ofthe brake ECU 200 for storing and executing the solenoid valve controlprogram, and so on. The communication condition control device 15 alsoserves as a normal use region velocity ratio reduction unit. Further, asolenoid valve control unit is constituted by the parts of the brake ECU200 for storing and executing the solenoid valve control program, and soon. Note that there are no limitations on respective structures of theregulator 90 and the servo pressure supply device 18. Moreover, theregulator 90 does not necessarily have to be provided, and the fluidpressure of the back surface chamber 16 may be controlled by controlperformed by the linear valve device 94. Furthermore, in the firstembodiment, the stroke velocity ratio is modified between the initialstage of the brake operation and normal use region, but a modificationtiming is not limited thereto. For example, the stroke velocity ratiomay be modified at a timing where the brake operating force reaches orexceeds a set force at which it may be determined that a large brakingforce is required.

The structure of the communication condition control device is notlimited to the structure described in the above embodiment, and astructure shown in FIGS. 6 and 7, for example, may be employed instead.All other parts are identical to the first embodiment, and thereforedescription thereof has been omitted. In this embodiment, as shown inFIGS. 6 and 7A, a communication condition control device 300 includes(a) the reservoir connection valve 58, and (b) a connection cutoffmechanism 302 that switches the communication conditions between theinter-piston chamber 30, the opposing chamber 38, and the reservoir 50mechanically. The connection cutoff mechanism 302 includes a housing310, and a movable member 312 provided to be capable of sliding relativeto the housing 310 in a direction of an axis Lt. An inter-piston chamberconnection port 313 to which the inter-piston chamber 30 is connected,an opposing chamber connection port 314 to which the opposing chamber 38is connected, a pilot pressure port 316 to which the fluid pressure inthe back surface chamber 16 is supplied as pilot pressure, and areservoir connection port 318 to which the reservoir 50 is connected areprovided in the housing 310 at intervals in the axis Lt direction.

The movable member 312 has a stepped shape in which an intermediatelarge diameter portion 330 having a large diameter is provided in anintermediate portion in the axis (Lt) direction, and a first smalldiameter portion 332 and a second small diameter portion 334respectively extending in the axis (Lt) direction are provided on eitherside of the intermediate large diameter portion 330. The first smalldiameter portion 332 extends in a T direction in FIG. 7A, while thesecond small diameter portion 334 extends in a TR direction (an oppositedirection to the T direction). A communication chamber 340 is formed onthe TR direction side (the second small diameter portion side) of theintermediate large diameter portion 330, and the inter-piston chamberconnection port 313 and the opposing chamber connection port 314 areopened on this side. Further, an elastic member (a blocking member) 342made of rubber or the like is disposed around an opening of theinter-piston chamber connection port 313 formed in the housing 310 toopen onto the communication chamber 340. When the second small diameterportion 334 contacts the elastic member 342, the opening of theinter-piston chamber connection port 313 into the communication chamber340 is blocked such that the inter-piston chamber 30 is cut off from theopposing chamber 38. In this sense, it may be considered that the secondsmall diameter portion 334, the opening of the inter-piston chamberconnection port 313 formed in the housing 310, the elastic member 342,and so on together constitute an inter-chamber connection cutoff valve.

A pilot pressure chamber 343 into which the pilot pressure port 316opens is formed on the opposite side of the intermediate large diameterportion 330 to the communication chamber 340 (i.e. on the T directionside). Further, a step surface 344 between the intermediate largediameter portion 330 and the first small diameter portion 332 of themovable member 312 receives fluid pressure from the pilot pressurechamber 343. Furthermore, a low pressure chamber 346 into which thereservoir connection port 318 opens is formed in a position opposing a Tdirection end surface 345 of the first small diameter portion 332, and aconnection passage 348 capable of connecting the low pressure chamber346 to the communication chamber 340 (i.e. having openings into both thelow pressure chamber 346 and the communication chamber 340) is formed inthe movable member 312. Meanwhile, an elastic member (a blocking member)350 is disposed in a position on the end surface 345 of the first smalldiameter portion 332 of the housing 310 that opposes an opening of theconnection passage 348. When the first small diameter portion 332 isseparated from the elastic member 350, the low pressure chamber 346 isconnected to the communication chamber 340 by the connection passage348. When the first small diameter portion 332 contacts the elasticmember 350, on the other hand, the connection passage 348 is blockedsuch that the low pressure chamber 346 is cut off from the communicationchamber 340. Hence, it may be considered that the first small diameterportion 332, the connection passage 348, the elastic member 350, and soon together constitute a reservoir cutoff valve. Note that the movablemember 312 is fitted to the housing 310 to be fluid-tight by theintermediate large diameter portion 330 and the first small diameterportion 332, and therefore the low pressure chamber 346, the pilotpressure chamber 343, and the communication chamber 340 are cut off fromeach other in a fluid-tight fashion. Further, a return spring 352 isprovided between the intermediate large diameter portion 330 and thehousing 310 in order to bias the movable member 312 in the T direction.

As shown in FIG. 7B, in the non-brake operation condition, the reservoirconnection valve 58 is open. The movable member 312 is in an originposition (a T direction movement end position) shown in the drawing, andtherefore the connection passage 348 is blocked. The communicationchamber 340 is cut off from the low pressure chamber 342, while theinter-piston chamber 30 and the opposing chamber 38 communicate via thecommunication chamber 340. This position of the movable member 312 willbe referred to as an inter-chamber connection position. Further, theopposing chamber 38 and the inter-piston chamber 30 communicate with thereservoir 50 via the reservoir connection valve 58.

[Initial Stage of Brake Operation]

When the brake pedal 26 is depressed, the reservoir connection valve 58is closed. In the connection cutoff mechanism 302, the fluid pressure ofthe back surface chamber 16 is supplied to the pilot pressure chamber344 such that TR direction force acts on the movable member 312. As longas the TR direction force is smaller than a set load of the returnspring 352, the movable member 312 remains in the inter-chamberconnection position shown in the drawing. The opposing chamber 38 andthe inter-piston chamber 30 communicate with each other but are cut offfrom the reservoir 50. This condition corresponds to the region RAf inFIG. 4. In the master cylinder 14, the stroke velocity ratio γa takes alarge value.

[Normal Use Region]

When the fluid pressure of the back surface chamber 16 increases suchthat the TR direction force exerted on the movable member 312 isincreased beyond the set load of the return spring 352 by fluid pressurein the pilot pressure chamber 344, the movable member 312 is moved inthe TR direction. When the first small diameter portion 332 separatesfrom the elastic member 350 and the second small diameter portion 334contacts the elastic member 342, the opening of the inter-piston chamberconnection port 313 is blocked such that the inter-piston chamber 30 iscut off from the opposing chamber 38. Further, the low pressure chamber346 communicates with the communication chamber 340 via the connectionpassage 348. As a result, the opposing chamber 38 communicates with thereservoir 50 via the connection passage 348. This position of themovable member 312 will be referred to as an inter-chamber cutoffposition. This condition corresponds to the region RBf in FIG. 4. Thestroke velocity ratio in the master cylinder 14 shifts to γb.

[When Abnormality Occurs in Electrical System]

When the current supply to the solenoid is stopped, the reservoirconnection valve 58 is opened. Further, when an abnormality occurs inthe electrical system, the fluid pressure in the back surface chamber 16cannot be raised sufficiently. Therefore, the TR direction force exertedon the movable member 312 cannot be increased beyond the set load of thereturn spring 352 by the fluid pressure in the back surface chamber 16,and as a result, the movable member 312 stays in the inter-chamberconnection position. The opposing chamber 38 and the inter-pistonchamber 30 communicate with each other, and communicate with thereservoir 50 via the reservoir connection valve 58. Similarly to thefirst embodiment, the stroke velocity ratio γc in the master cylinder 14reaches 1.

Hence, according to the communication condition control device 300according to the second embodiment, in the non-brake operation conditionand when an abnormality occurs in the electrical system, the opposingchamber 38 and the inter-piston chamber 30 communicate with thereservoir 50 via the reservoir connection valve 58, while in the normaluse region, the opposing chamber 38 communicates with the reservoir 50via the connection cutoff mechanism 302. During the brake operation,therefore, the opposing chamber 38 and the reservoir 50 can be switchedbetween a communicative condition and a cutoff condition withoutcontrolling the solenoid of the reservoir connection valve 58. As aresult, when a brake operation is performed while the electrical systemis normal, the stroke velocity ratio in the master cylinder 14 can bemodified in two stages.

The communication condition control device may also be structured asshown in FIGS. 8 and 9. All other structures are identical to the firstembodiment, and therefore description thereof has been omitted. In thisembodiment, a communication condition control device 380 includes aconnection cutoff mechanism 382, and a flow limitation device 384provided between the reservoir 50 and the opposing chamber 38. As shownin FIG. 9A, in the connection cutoff mechanism 302 according to thesecond embodiment, the back surface chamber 16 is connected to the pilotpressure port 316, whereas in the connection cutoff mechanism 382, thepressure chamber 29 is connected to the pilot pressure port 316. Theflow limitation device 384 includes (i) a check valve 392 that allowsthe working fluid to flow from the reservoir 50 into the opposingchamber 38 but prohibits the working fluid from flowing in reverse, and(ii) a relief valve 390 that allows the working fluid to flow from theopposing chamber 38 into the reservoir 50 when the fluid pressure in theopposing chamber 38 exceeds the fluid pressure in the reservoir 50 by atleast a set relief pressure, wherein the check valve 392 and the reliefvalve 390 are provided in parallel. The check valve 390 is provided toprevent negative pressure in the opposing chamber 38, and returnsworking fluid to the opposing chamber 38 from the reservoir 50 when theoperation of the brake pedal 26 is released.

As shown in FIG. 9B, in the non-brake operation condition, theinter-piston chamber 30 and the opposing chamber 38 communicate witheach other and are connected to the reservoir 50 via the flow limitationdevice 384. Hence, the inter-piston chamber 30, the opposing chamber 38,and the reservoir 50 are substantially communicative.

[Initial Stage of Brake Operation]

Even when the brake pedal 26 is depressed, the movable member 312 staysin the inter-chamber communication condition shown in the drawing aslong as the fluid pressure in the pressure chamber 29 remains low. Sincethe opposing chamber 38 and the inter-piston chamber 30 arecommunicative, the fluid pressure in the opposing chamber 38 is suppliedto the inter-piston chamber 30. As a result, the fluid pressure in theopposing chamber 38 does not increase beyond the set relief pressure,and therefore the opposing chamber 38 is substantially cut off from thereservoir 50. This condition corresponds to the region RAf in FIG. 4.

[Normal Use Region]

When the fluid pressure in the pressure chamber 29 increases such thatthe TR direction force exerted on the movable member 312 increasesbeyond the set load of the return spring 352, the movable member 312 ismoved to the inter-chamber cutoff position. The opposing chamber 38 iscut off from the inter-piston chamber 30, but communicates with thereservoir 50 via the connection passage 348. This condition correspondsto the region RBf in FIG. 4.

[When Abnormality Occurs in Electrical System]

Even when an abnormality occurs in the electrical system, fluid pressureis generated in the pressure chambers 28, 29 of the master cylinder 14by a manual operation. As long as the fluid pressure in the pressurechamber 29 remains low, the movable member 312 stays in theinter-chamber connection position, but when the fluid pressure in thepressure chamber 29 increases as a result of the manual operation, thefluid pressure in the pilot pressure chamber 343 increases. When the TRdirection force exerted on the movable member 312 increases beyond theset load (a set value) of the return spring 352, the movable member 312is moved to the inter-chamber cutoff position, and as a result, theinter-piston chamber 30 is cut off from the opposing chamber 38. Theopposing chamber 38 communicates with the reservoir 50 via theconnection passage 348. Since the inter-piston chamber 30 is closed, thestroke velocity ratio γc reaches (a3/a2), which is larger than thevalues thereof in the first and second embodiments.

Hence, in this embodiment, it is possible during the brake operation toswitch between a condition in which the opposing chamber 38 and theinter-piston chamber 30 communicate with each other but are cut off fromthe reservoir 50 and a condition in which the opposing chamber 38communicates with the reservoir 50 while the inter-piston chamber 30 iscut off from both the reservoir 50 and the opposing chamber 38 eventhough the communication condition control device 380 does not include asolenoid valve. Further, when an abnormality occurs in the electricalsystem, the inter-piston chamber 30 is cut off, and therefore the strokevelocity ratio can be set at a value larger than 1, enabling a reductionin the operating stroke of the brake pedal 26. Note that the check valve392 does not necessarily have to be provided, and a cap seal providedbetween the reservoir port of the master cylinder 14 and the opposingchamber 38 may be used instead. An example of this will be described asa fourth embodiment.

The communication condition control device may also be structured asshown in FIG. 10. As shown conceptually in FIG. 11A, a communicationcondition control device 400 includes (i) a reservoir connection valve410 constituted by a solenoid valve provided between the inter-pistonchamber 30 and the reservoir 50, (ii) a connection valve 412 that isprovided between the inter-piston chamber 30 and the opposing chamber38, and is switched to an open condition when the fluid pressure in theopposing chamber 38 is higher, thereby permitting a bidirectional flow,and switched to a closed condition when the fluid pressure in theinter-piston chamber 30 is higher, and (iii) a flow limitation device414 provided between the opposing chamber 38 and the reservoir 50. Theflow limitation device 414 includes (a) a check valve 416 that allowsthe working fluid to flow from the reservoir 50 into the opposingchamber 38 but prohibits the working fluid from flowing in reverse, and(b) a relief valve 418 that allows the working fluid to flow from theopposing chamber 38 into the reservoir 50 when the fluid pressure in theopposing chamber 38 exceeds the fluid pressure in the reservoir 50 by atleast a set relief pressure, wherein the check valve 416 and the reliefvalve 418 are provided in parallel. The reservoir connection valve 410is a normally open valve that is open when no current is supplied to thesolenoid thereof. Further, as shown in FIG. 10, in this embodiment, theconnection valve 412 and the check valve 416 are provided in an interiorof a master cylinder 420. The check valve 416 serves as a cap sealprovided between the reservoir 50 and the opposing chamber 38, while theconnection valve 412 is provided in a connection passage 424 formed in arear small diameter portion 422 of a pressure piston 421 to connect theinter-piston chamber 30 and the opposing chamber 38. All other parts areidentical to the first embodiment, and therefore description thereof hasbeen omitted.

As shown in FIG. 11B, in the non-brake operation condition, thereservoir connection valve 410 is open, and therefore the inter-pistonchamber 30 communicates with the reservoir 50. Further, the opposingchamber 38 communicates with the reservoir 50 either via the check valve416 or via the connection valve 412, the inter-piston chamber 30, andthe reservoir connection valve 410, and therefore the opposing chamber38 and the reservoir 50 are substantially communicative.

[Initial Stage of Brake Operation]

When the brake pedal 26 is depressed, the reservoir connection valve 410is closed, and therefore the inter-piston chamber 30 is cut off from thereservoir 50. Meanwhile, the fluid pressure in the back surface chamber16 increases, causing the advancement direction force exerted on thepressure piston 25 to increase, and when the fluid pressure in theopposing chamber 38 increases, the working fluid is permitted to flowfrom the opposing chamber 38 into the inter-piston chamber 30 throughthe connection valve 412. Accordingly, advancement of the pressurepiston 25 is permitted. The fluid pressure in the opposing chamber 38does not increase beyond the set relief pressure, and therefore theopposing chamber 38 is substantially cut off from the reservoir 50. Thiscondition corresponds to the region RAf in FIG. 4.

[Normal Use Region]

When the fluid pressure in the back surface chamber 16 increases suchthat the fluid pressure in the opposing chamber 38 rises beyond the setrelief pressure, the working fluid flows from the opposing chamber 38into the reservoir 50 through the relief valve 418. When the fluidpressure in the inter-piston chamber 30 increases beyond the fluidpressure in the opposing chamber 38, the connection valve 412 is closed,and therefore the inter-piston chamber 30 is cut off from both theopposing chamber 38 and the reservoir 50. This condition corresponds tothe region RBf in FIG. 4.

[When Abnormality Occurs in Electrical System]

When the current supply to the solenoid is stopped, the reservoirconnection valve 410 is opened. Hence, the inter-piston chamber 30 andthe opposing chamber 38 both communicate with the reservoir 50. As aresult, the input piston 22 and the pressure piston 25 are movedintegrally such that the stroke velocity ratio γc reaches 1.

In this embodiment, therefore, the stroke velocity ratio can be switchedin two stages in the master cylinder 420 using a simple structurewithout a connection cutoff mechanism.

The structure of the servo pressure supply device is not limited to thatof the embodiments described above, and a structure shown in FIGS. 12and 13 may be employed instead. All other parts are identical to thefirst embodiment, and therefore description thereof has been omitted. Asshown in FIGS. 12 and 13A, a servo pressure supply device 450 includes aregulator 460, the high pressure source 92, a linear valve device 462, aservo fluid pressure sensor 464 that detects the fluid pressure in theback surface chamber 16, and so on. The regulator 460 is providedbetween the back surface chamber 16, the high pressure source 92, thelinear valve device 462, and the reservoir 50, and in the regulator 460,the servo pressure supplied to the back surface chamber 16 is controlledby control performed by the linear valve device 462 using the fluidpressure of the high pressure source 92. The regulator 460 includes ahousing 500, and a plurality of movable members 502 to 506 fitted to thehousing 500 in series so as to be fluid-tight and capable of sliding. Anoutput port 510 connected to the back surface chamber 16, a highpressure port 512 connected to the high pressure source 92, a lowpressure port 514 connected to the reservoir 50, a linear pressure port516 connected to the linear valve device 462, and a pilot pressure port518 connected to the pressure chamber 29 are provided in the housing 500at intervals in a direction of an axis (Lr).

The movable member 502 can be moved by the fluid pressure of the pilotpressure port 518. A movable member 504 has a stepped shape including asmall diameter portion 520 and a large diameter portion 522, wherein alarge diameter portion side end surface serves as a pressure receivingsurface for receiving fluid pressure from the linear pressure port 516,or in other words fluid pressure controlled by the linear valve device462. Thus, the movable member 504 can be moved by the fluid pressurecontrolled by the linear valve device 462. An axial direction passage524 and an output passage 526 serving as a radial direction passage areformed in a mutually communicative condition in the movable member 506.The output passage 526 communicates with the output port 510. Further,the movable member 506 has a stepped shape including a small diameterportion 528 and a large diameter portion 530, wherein an annular grooveportion 532 provided in an outer peripheral surface of the smalldiameter portion 528 to extend in a parallel direction to the axis Lrcommunicates with the high pressure port 512. A step portion (a valveelement) 534 between the small diameter portion 528 and the largediameter portion 530 and a step portion (a valve seat) 536 provided inthe housing 500 together constitute a high pressure supply valve 538. Byopening and closing the high pressure supply valve 538, the annulargroove portion 532 is connected to and cut off from the output port 510.The high pressure supply valve 538 is biased to a closed condition by aspring 540 provided between the movable member 506 and the housing 500.Further, the small diameter portion 520 of the movable member 504 ispositioned inside the axial direction passage 524 of the movable member506, whereby a step portion (a valve element) 544 between the smalldiameter portion 520 and the large diameter portion 522 of the movablemember 504 and an opening edge portion (a valve seat) 546 of the axialdirection passage 524 of the movable member 506 together constitute alow pressure cutoff valve 548. By opening and closing the low pressurecutoff valve 548, the low pressure port 514 is connected to and cut offfrom the output port 510. The low pressure cutoff valve 548 is biased toan open condition by a spring 550 provided between the movable member504 and the movable member 506. An elastic member (a member formed fromrubber, for example) 552 is provided between an end portion of themovable member 506 on an opposite side to the movable member 504 and thehousing 500. When the elastic member 552 undergoes elastic deformation,the movable member 506 is permitted to move in a direction of an arrow P(movement in a direction for switching the high pressure supply valve538 to an open condition).

The linear valve device 462 includes a pressure increasing linear valve570 provided between the high pressure source 92 and the linear pressureport 516, and a pressure reducing linear valve 572 provided between thelinear pressure port 516 and the reservoir 50. Respective front-reardifferential pressures of the pressure increasing linear valve 570 andthe pressure reducing linear valve 572 can be controlled to magnitudescorresponding to amounts of current supplied to respective solenoidsthereof. Further, the pressure increasing linear valve 570 is a normallyclosed valve that is closed when no current is supplied to the solenoidthereof, while the pressure reducing linear valve 572 is a normally openvalve that is open when no current is supplied to the solenoid thereof.By controlling the pressure increasing linear valve 570 and the pressurereducing linear valve 572, the fluid pressure of the linear pressureport 516 is controlled to a desired magnitude. Furthermore, the fluidpressure in the pressure chamber 29 is supplied to the pilot pressureport 518.

In the servo pressure supply device 450, the currents supplied to thesolenoids of the linear valve device 462 are controlled such that theservo pressure, or in other words the fluid pressure actually outputfrom the output port 510, which is detected by the servo pressure sensor464, approaches a target fluid pressure. By controlling the fluidpressure of the linear pressure port 516, the high pressure supply valve538 and the low pressure cutoff valve 548 are opened and closed, and asa result, the servo pressure approaches the target fluid pressure. Inthis embodiment, as shown in FIG. 13B, the target fluid pressure of theservo pressure is determined such that in the initial stage of the brakeoperation, a gain takes a large value relative to the brake operatingforce, whereas in the normal use region, the gain takes a small valuerelative to the brake operating force. An increase gradient of the fluidpressure actually output from the output port 510 is therefore large inthe initial stage of the brake operation and smaller in the normal useregion.

Note that in the regulator 460, the inter-piston chamber 30 may beconnected to the pilot pressure port 518. Either the fluid pressure ofthe pressure chamber 29 or the fluid pressure of the inter-pistonchamber 30 may be used as the pilot pressure, and in both cases, fluidpressure corresponding to the brake operating force can be used.

In this embodiment, as shown in FIG. 14, in a master cylinder 600, theeffective pressure receiving surface area a2 of the pressure piston 25relative to the inter-piston chamber 30 and the effective pressurereceiving surface area a3 of the input piston 22 are substantiallyidentical (a2=a3). Therefore, as shown in FIG. 15, the stroke velocityratio in the region RAf is smaller than that of the first embodiment,while the stroke velocity ratio in the region RBf is 1. By modifying therespective magnitudes of the effective pressure receiving surface areasof the pressure piston 25 and the input piston 22 relative to theinter-piston chamber 30 in this manner, the stroke velocity ratio can bemodified appropriately.

In this embodiment, as shown in FIG. 16, in a master cylinder 650, thesurface area of the step surface 36 of the pressure piston 25, or inother words the effective pressure receiving surface area a1 of the partopposing the opposing chamber 38, and the effective pressure receivingsurface area a2 of the part opposing the inter-piston chamber 30 aresubstantially identical (a1≅a2). Therefore, as shown in FIG. 17, in theinitial stage of the brake operation, the working fluid is supplied fromthe opposing chamber 38 to the inter-piston chamber 30 such thatadvancement of the input piston 22 is suppressed. As a result, the ratiobetween the stroke velocity of the input piston 22 and the strokevelocity of the pressure piston 25 becomes extremely large in a regionFAf so as to be theoretically infinite. By greatly increasing the strokevelocity ratio in the initial stage of the brake operation in thismanner, an initial response delay in the brake can be suppressed evenfurther.

There are no limitations on the structure of the hydraulic brake circuitand so on, and in addition to the embodiments described above, theinvention may be implemented in various other modified and amendedembodiments on the basis of the knowledge of persons skilled in the art.

A period required for the input piston to move from the retreat endposition to the advancement end position may mean “a period of a singlecontinuous operation of the brake operating member”. The stroke may bean amount by which the input piston moves from the retreat end position,and the stroke velocity may be an amount of variation in the strokewithin a set time. The stroke velocity ratio may be modified in twostages, three or more stages, or continuously.

The master cylinder apparatus may include the back surface chamberformed to the rear of the pressure receiving surface of the pressurepiston such that the pressure piston can be caused to advance relativeto the input piston by the fluid pressure in the back surface chamber.The pressure receiving surface is often provided rearward of the largediameter portion of the pressure piston.

In the master cylinder apparatus described in item (7), thecommunication condition control device includes (a) a flow limitationdevice having (a-i) a relief valve that allows the working fluid to flowfrom the opposing chamber into the reservoir when the fluid pressure inthe opposing chamber exceeds the fluid pressure in the reservoir by aset relief pressure, but prevents the working fluid from flowing inreverse and (a-ii) a check valve that allows the working fluid to flowfrom the reservoir into the opposing chamber but prevents the workingfluid from flowing in reverse, wherein the relief valve and the checkvalve are provided in parallel between the opposing chamber and thereservoir, (b) a connection valve that is provided between the opposingchamber and the inter-piston chamber, switched to an open condition inwhich a bidirectional flow is permitted when the fluid pressure in theopposing chamber is higher than the fluid pressure in the inter-pistonchamber, and switched to a closed condition when the fluid pressure inthe inter-piston chamber is higher than the fluid pressure in theopposing chamber, and (c) a reservoir/inter-piston chamber connectionvalve constituted by a normally open solenoid valve provided between theinter-piston chamber and the reservoir. When the brake operating memberis operated, the reservoir/inter-piston chamber connection valve isclosed. (i) When a force generated by the fluid pressure in the backsurface chamber acts on the pressure piston, the fluid pressure in theopposing chamber increases. Since the working fluid is allowed to flowfrom the opposing chamber into the inter-piston chamber, the pressurepiston is allowed to advance. Therefore, the opposing chamber and theinter-piston chamber are substantially communicative. Further, when theworking fluid flows from the opposing chamber into the inter-pistonchamber, the fluid pressure in the opposing chamber does not reach orexceed the set relief pressure. Therefore, the opposing chamber issubstantially cut off from the reservoir. Hence, the opposing chamberand the inter-piston chamber are substantially communicative, theinter-piston chamber is cut off from the reservoir, and the opposingchamber is substantially cut off from the reservoir. (ii) When the fluidpressure in the back surface chamber increases further such that thefluid pressure in the opposing chamber increases beyond the set reliefpressure, the working fluid flows out of the opposing chamber into thereservoir via the relief valve. As a result, the fluid pressure in theinter-piston chamber increases beyond the fluid pressure in the opposingchamber, whereby the connection valve is switched to the closedcondition. Hence, the inter-piston chamber is substantially cut off fromthe opposing chamber and also cut off from the reservoir, while theopposing chamber and the reservoir are substantially communicative.(iii) When an abnormality occurs in the electrical system, thereservoir/inter-piston chamber connection valve opens, and therefore theinter-piston chamber communicates with the reservoir. Further, the fluidpressure in the opposing chamber increases beyond the fluid pressure inthe inter-piston chamber so that the working fluid is permitted to flowfrom the opposing chamber into the inter-piston chamber via theconnection valve, and therefore the opposing chamber and theinter-piston chamber are substantially communicative. Moreover, theopposing chamber substantially communicates with the reservoir via theinter-piston chamber. The check valve supplies working fluid to theopposing chamber when the operation of the brake operating member isreleased or the like, for example, and therefore, with the check valve,the opposing chamber is favorably prevented from entering negativepressure.

In the master cylinder apparatus including the input piston that iscaused to advance by operating the brake operating member, and thepressure piston that is provided coaxially with the input piston to becapable of moving relative to the input piston, the input piston isdisposed opposite the pressure piston via the inter-piston chamber, andthe pressure piston has a stepped shape including the large diameterportion and the front small diameter portion that has a smaller diameterthan the large diameter portion and is provided in front of the largediameter portion. The master cylinder apparatus also includes thecommunication condition control device having (i) the inter-chamberconnection cutoff valve constituted by a normally open solenoid valveprovided between the opposing chamber, which is provided in front of astep surface between the large diameter portion and the front smalldiameter portion, and the inter-piston chamber, (ii) the reservoirconnection valve constituted by a normally open solenoid valve providedbetween the opposing chamber and the reservoir, and (iii) the solenoidvalve control unit that controls the communication conditions betweenthe opposing chamber, the inter-piston chamber, and the reservoir bycontrolling the reservoir connection valve and the inter-chamberconnection cutoff valve. Note that the communication condition controldevice may be provided separately to the master cylinder or as part ofthe constituent elements of the master cylinder.

The back surface fluid pressure control device may include (i) thehousing in which at least the output port connected to the back surfacechamber, the high pressure port connected to the high pressure source,and the low pressure port connected to the reservoir are formed, (ii)the spool that is disposed in the housing to be capable of relativemovement and can control the fluid pressure output from the output portby connecting the output port selectively to the high pressure port orthe low pressure port, and (iii) the regulator having a spool movingdevice which, when a force that acts on the spool and is determined bythe operating condition of the brake operating member reaches or exceedsa predetermined set value while the spool is in a pressure increasingposition in which the output port is cut off from the low pressure portand connected to the high pressure port, moves the output port to anon-pressure increasing position in which the output port is cut offfrom the high pressure port. While the force determined by the operatingcondition of the brake operating member remains smaller than the setvalue, or in other words in the initial stage of the brake operation,the spool is in the pressure increasing position, and therefore thefluid pressure in the back surface chamber can be increased on a largegradient.

The invention claimed is:
 1. A master cylinder apparatus comprising: aninput piston that is configured to move forward by operating a brakeoperating member; and a pressure piston that is provided in front of theinput piston configured to move relative to the input piston, whereinthe input piston is disposed opposite the pressure piston via aninter-piston chamber, the pressure piston includes a large diameterportion, a front small diameter portion that is provided in front of thelarge diameter portion and has a smaller diameter than the largediameter portion, and a step constituted by the large diameter portionand the front small diameter portion, the surface constituting the stepbetween the large diameter portion and the front small diameter portionof the pressure piston forms an opposing chamber, a first effectivepressure receiving surface area of the pressure piston relative to theopposing chamber is equal to or smaller than a second effective pressurereceiving surface are thereof relative to the inter-piston chamber, andthe master cylinder apparatus is configured to: modify a stroke velocityratio, which is a ratio between a stroke velocity of the pressure pistonand a stroke velocity of the input piston, in at least two stages whilethe input piston moves from a rear end portion position to a front endportion position, set the stroke velocity ratio at 1 when an abnormalityoccurs in the master cylinder apparatus, and set the stroke velocityratio small when a stroke of the input piston is large, compared to thestroke velocity ratio when the stroke of the input piston is small. 2.The master cylinder apparatus according to claim 1, wherein the mastercylinder apparatus is further configured to modify the stroke velocityratio in the at least two stages when the stroke velocity ratio equalsor exceeds
 1. 3. The master cylinder apparatus according to claim 1,wherein the master cylinder apparatus further comprising a communicationcondition control device that is provided between the inter-pistonchamber, the opposing chamber, and a reservoir and that is configured tocontrol communication conditions between the inter-piston chamber, theopposing chamber, and the reservoir so as to switch between aninter-chamber connection condition in which the opposing chamber and theinter-piston chamber communicate with each other but are cut off fromthe reservoir, an inter-chamber cutoff condition in which the opposingchamber is cut off from the inter-piston chamber, the inter-pistonchamber is cut off from the reservoir, and the opposing chambercommunicates with the reservoir, and a reservoir connection condition inwhich both the opposing chamber and the inter-piston chamber communicatewith the reservoir.
 4. The master cylinder apparatus according to claim3, wherein the communication condition control device comprises: aninter-chamber connection cutoff valve constituted by a normally opensolenoid valve provided between the opposing chamber and theinter-piston chamber; a reservoir connection valve constituted by anormally open solenoid valve provided between the opposing chamber andthe reservoir; and a solenoid valve control unit that controls thecommunication conditions between the opposing chamber, the inter-pistonchamber, and the reservoir by controlling the reservoir connection valveand the inter-chamber connection cutoff valve.
 5. The master cylinderapparatus according to claim 3, wherein the communication conditioncontrol device comprises: a connection cutoff mechanism that is providedbetween the opposing chamber and the inter-piston chamber and reservoirand includes a movable member that is operated by a pilot pressure, amagnitude of which is determined through electric control, and that isconfigured to move between a communication position in which theopposing chamber and the inter-piston chamber communicate with eachother but are cut off from the reservoir, and a cutoff position in whichthe opposing chamber communicates with the reservoir but theinter-piston chamber is cut off from both the opposing chamber and thereservoir; and a reservoir connection valve constituted by a normallyopen solenoid valve provided between the opposing chamber and thereservoir.
 6. The master cylinder apparatus according to claim 1,further comprising a back surface pressure control device including apower pressure source that is operated by a supply of power and isconfigured to output a fluid at a predetermined pressure, and aregulator that controls a pressure exerted on a back surface chamber,which is provided rearward of a pressure receiving surface of thepressure piston, to a magnitude corresponding to an operating conditionof the brake operating member by using the pressure output by the powerpressure source.
 7. A master cylinder apparatus comprising: an inputpiston that is configured to move forward by operating a brake operatingmember; and a pressure piston that is provided coaxially with the inputpiston and configured to move relative to the input piston, and disposedopposite the input piston via an inter-piston chamber, and has a steppedshape including a large diameter portion and a front small diameterportion that has a smaller diameter than the large diameter portion andis provided in front of the large diameter portion; wherein acommunication condition control device is provided between an opposingchamber, which is provided in front of a step surface between the largediameter portion and the front small diameter portion, the inter-pistonchamber and a reservoir, the communication condition control devicebeing configured to switch communication conditions among the opposingchamber, the inter-piston chamber, and the reservoir between at least acondition in which the opposing chamber and the inter-piston chambercommunicate with each other but are cut off from the reservoir, acondition in which the opposing chamber is cut off from the inter-pistonchamber, the opposing chamber communicates with the reservoir, and theinter-piston chamber is cut off from the reservoir, and a condition inwhich both the opposing chamber and the inter-piston chamber communicatewith the reservoir.
 8. A master cylinder apparatus comprising: an inputpiston that is configured to move forward by operating a brake operatingmember; and a pressure piston that is provided in front of the inputpiston configured to move relative to the input piston, wherein theinput piston is disposed opposite the pressure piston via aninter-piston chamber, the pressure piston includes a large diameterportion, and a front small diameter portion that is provided in front ofthe large diameter portion and has a smaller diameter than the largediameter portion, a surface area of the large diameter portion of thepressure piston on which pressure is received from a front side is equalto or smaller than a surface area of the pressure piston on whichpressure is received from the inter-piston chamber side, and the mastercylinder apparatus is configured to: modify a stroke velocity ratio,which is a ratio between a stroke velocity of the pressure piston and astroke velocity of the input piston, in at least two stages while theinput piston moves from a rear end portion position to a front endportion position, and set the stroke velocity ratio at 1 when anabnormality occurs in the master cylinder apparatus, and set the strokevelocity ratio small when a stroke of the input piston is large,compared to the stroke velocity ratio when the stroke of the inputpiston is small.
 9. A master cylinder apparatus comprising: an inputpiston that is configured to move forward by operating a brake operatingmember; and a pressure piston that is provided in front of the inputpiston configured to move relative to the input piston, wherein theinput piston is disposed opposite the pressure piston via aninter-piston chamber, the pressure piston includes a large diameterportion, a front small diameter portion that is provided in front of thelarge diameter portion and has a smaller diameter than the largediameter portion, and a step constituted by the large diameter portionand the front small diameter portion, the surface constituting the stepbetween the large diameter portion and the front small diameter portionof the pressure piston forms an opposing chamber, the master cylinderapparatus is configured to: modify a stroke velocity ratio, which is aratio between a stroke velocity of the pressure piston and a strokevelocity of the input piston, in at least two stages while the inputpiston moves from a rear end portion position to a front end portionposition, set the stroke velocity ratio at 1 when an abnormality occursin the master cylinder apparatus, and set the stroke velocity ratiosmall when a stroke of the input piston is large, compared to the strokevelocity ratio when the stroke of the input piston is small, and themaster cylinder apparatus further comprising a communication conditioncontrol device that is provided between the inter-piston chamber, theopposing chamber, and a reservoir and that is configured to controlcommunication conditions between the inter-piston chamber, the opposingchamber, and the reservoir so as to switch between an inter-chamberconnection condition in which the opposing chamber and the inter-pistonchamber communicate with each other but are cut off from the reservoir,an inter-chamber cutoff condition in which the opposing chamber is cutoff from the inter-piston chamber, the inter-piston chamber is cut offfrom the reservoir, and the opposing chamber communicates with thereservoir, and a reservoir connection condition in which both theopposing chamber and the inter-piston chamber communicate with thereservoir.
 10. The master cylinder apparatus according to claim 9,wherein the communication condition control device comprises: aninter-chamber connection cutoff valve constituted by a normally opensolenoid valve provided between the opposing chamber and theinter-piston chamber; a reservoir connection valve constituted by anormally open solenoid valve provided between the opposing chamber andthe reservoir; and a solenoid valve control unit that controls thecommunication conditions between the opposing chamber, the inter-pistonchamber, and the reservoir by controlling the reservoir connection valveand the inter-chamber connection cutoff valve.
 11. The master cylinderapparatus according to claim 9, wherein the communication conditioncontrol device comprises: a connection cutoff mechanism that is providedbetween the opposing chamber and the inter-piston chamber and reservoirand includes a movable member that is operated by a pilot pressure, amagnitude of which is determined through electric control, and that isconfigured to move between a communication position in which theopposing chamber and the inter-piston chamber communicate with eachother but are cut off from the reservoir, and a cutoff position in whichthe opposing chamber communicates with the reservoir but theinter-piston chamber is cut off from both the opposing chamber and thereservoir; and a reservoir connection valve constituted by a normallyopen solenoid valve provided between the opposing chamber and thereservoir.